A particularly useful class of shaped thermoplastic articles are those which are formed into sheets which are thermoformable so that cavities can be formed in the sheets to receive a product to be packaged. To close the package a sheet is laid over the filled cavity and sealed to the perimeter of the cavity.
In recent years the trend has been to produce thermoformable sheets which are laminates of several different polymeric materials in order to combine the desirable properties of each polymeric material into one film, for example, individual layers of materials having excellent strength or toughness, heat sealability, moisture and gas barriers, etc. are combined to form the final laminate. One way of producing such a laminate is to coextrude the polymeric layers so that they are melt joined within the coextrusion die and in many instances this is less costly and produces better bonding than producing the layers separately and then adhesively bonding them together. In other instances, it may be desirable to produce a film laminate in a coextrusion process which has many general usages including the capability to be adhesively laminated to another layer. Accordingly, one object of the present invention is to provide a coextruded film laminate which has excellent sealing properties and which can be laminated to another polymeric film or film laminate.
One of the problems often encountered in thermoplastic film laminates is that undesirable delamination occurs so that the layers or plies separate under certain conditions. Accordingly, it is another object of the present invention to provide a thermoplastic film laminate which is highly resistant to internal delamination, i.e., the layers of the laminate show superior layer-to-layer bonds within the laminate.
One class of polymeric materials, specifically the ionomer resins generally sold under the brand name of "Surlyn" have achieved widespread and general acceptance as excellent sealants and are also approved for usage in food packaging. However, the Surlyn brand materials are expensive and add significantly to the cost of thermoplastic laminates. Accordingly, another object of the present invention is to utilize the benefits of ionomeric resins but reduce the cost of the resulting laminate.